Effects of magnesium and chloride ions on light-induced electron transport in membrane fragments from a blue-green alga

Abstract

The effects of magnesium and chloride ions on photosynthetic electron transport were investigated in membrane fragments of a blue-green alga, Nostoc muscorum (Strain 7119), noted for their stability and high rates of electron transport from water or reduced dichlorophenolindophenol to NADP +. Magnesium ions were required not only for light-induced electron transport from water to NADP + but also for protection in the dark of the integrity of the water-photooxidizing system (Photosystem II). Membrane fragments suspended in the dark in a medium lacking Mg 2+ lost the capacity to photoreduce NADP + with water on subsequent illumination. Chloride ions could substitute, but less effectively, for each of these two effects of magnesium ions. By contrast, the photoreduction of NADP + by DCIPH 2 was independent of Mg 2+ (or Cl −) for the protection of the electron transport system in the dark or during the light reaction proper. Furthermore, high concentrations of MgCl 2 produced a strong inhibition of NADP + photoreduction with DCIPH 2 without significantly affecting the rate of NADP + photoreduction with water. The implications of these findings for the differential involvement of Photosystem I and Photosystem II in the photoreduction of NADP + with different electron donors are discussed.